Celestron EdgeHD White Paper - First Light Optics

The Celestron EdgeHD 

A flexible imaging platform... 

...at an affordable price. 

Superior flat-field, coma-free imaging! 

by the Celestron Engineering Team 

2835 Columbia Street 

Torrance, CA 90503 

www.celestron.com 

Ver. 09-2012, For release in September 2012.

The Celestron EdgeHD 

A Flexible Imaging Platform 

at an Affordable Price 

By the Celestron Engineering Team 

Abstract: The Celestron EdgeHD is an advanced, flat-field, aplanatic series of telescopes 

for visual observation and imaging with astronomical CCD cameras and full-frame digital 

SLR cameras. This paper describes the development goals, design decisions, optical 

performance, and their practical realization in 8-, 9.25-, 11-, and 14-inch apertures. We 

include cross-sections of the EdgeHD series, comparative spot diagrams for the EdgeHD 

and competing “coma-free” Schmidt-Cassegrain designs, a table with specifications for 

visual and imaging, graphics showing how to place sensors at the optimum back-focus 

distance, and details on the construction and testing of the EdgeHD telescope series. 

1. Introduction 

The ʺclassicʺ Schmidt‐Cassegrain telescope manufactured 

by Celestron served an entire generation of 

observers and astrophotographers. With the advent of 

wide‐field and ultra‐wide field eyepieces, large‐format 

CCD cameras, and full‐frame digital SLR cameras, the 

inherent drawbacks of the classic SCT called for a new 

design. The EdgeHD is that new design. The EdgeHD 

offers clean, diffraction‐limited images for high‐power 

observation of the planets and the moon. And, as an 

aplanatic flat‐field astrograph, the EdgeHDʹs optics 

provide tight, round, edge‐to‐edge star images over a 

wide, 42 mm diameter, flat field of view for stunning 

color, monochrome, and narrow‐band imaging of 

deep‐sky objects. 

2. Setting Goals for the EdgeHD Telescope 

The story of the EdgeHD began with our setting 

performance goals, quality goals, and price goals. Like 

the classic SCT, the new Celestron optic would need to 

be light and compact. Optically, we set twin goals: the 

new telescope would be capable of extraordinary 

wide‐field viewing with advanced eyepiece designs, 

and it would be capable of sharp‐to‐the‐edge astrophotography 

with advanced digital SLR cameras and 

astronomical CCD cameras. Cost wise, we wanted to 

leverage Celestronʹs proven ability to manufacture 

high‐performance telescopes at a user‐friendly price 

point. In short, our goal was to offer observers a flexible 

imaging platform at a very affordable price. 

Given an unlimited budget, engineering high‐performance 

optics is not difficult. The challenge 

Celestron accepted was to control the price, complexity, 

and cost of manufacture without compromise to 

optical performance. We began with a comprehensive 

review of the classic SCT and possible alternatives. 

Our classic SCT has three optical components: a 

spherical primary mirror, a spherical secondary mirror, 

and a corrector plate with a polynomial curve. As 

every amateur telescope maker and professional optician 

knows, a sphere is the most desirable optical figure. 

In polishing a lens or mirror, the work‐piece 

moves over a lap made of optical pitch that slowly conforms 

to the glass surface. Geometrically, the only surfaces 

that can slide freely against one another are 

spheres: any spot that is low relative to the common 

spherical surface receives no wear; any spot that is high 

relative is worn off. Spherical surfaces result almost 

automatically. 

A skilled optician in a well‐equipped optical shop 

can reliably produce near‐perfect spherical surfaces. 

Furthermore, by comparing an optical surface against a 

matchplate—a precision reference surface—departures 

in both the radius and sphericity can be quickly 

assessed. In forty years of manufacturing its classic 

Schmidt Cassegrain telescope, Celestron had fully 

mastered the art of making large numbers of essentially 

perfect spherical primary and secondary mirrors. 

In addition, Celestron’s strengths included the production 

of Schmidt corrector plates. In the early 1970s, 

Tom Johnson, Celestronʹs founder, perfected the necessary 

techniques. Before Johnson, corrector plates like 

that on the 48‐inch Schmidt camera on Palomar Mountain 

cost many long hours of skilled work by master 

opticians. Johnsonʹs innovative production methods 

made possible the volume production of a complex 

and formerly expensive optical component—and triggered 

the SCT Revolution of the 1970s. 

The EdgeHD by Celestron 2

Edge HD 800 

Edge HD 925 

Edge HD 1100 

Edge HD 1400 

Figure 1. Celestron’s EdgeHD series consists of four 

aplanatic telescopes with 8-, 9.25-, 11-, 14-inch 

apertures. The optical design of each instrument has 

been individually optimized to provide a focal plane 

that is coma-free, flat, and produces sharp images to 

the edge of the view with minimal vignetting. 


Optical Aberrations 

For those not familiar with the art of optical 

design, this brief primer explains what 

aberrations are and how they appear 

in a telescopic image. 

Off-Axis Coma 

Coma is an off-axis aberration that results 

when the rays from successive zones are 

displaced outward relative to the principal 

(central) ray. A star image with coma appears 

to have wispy “hair” or little “wings” extending 

from the image. In a coma-free optical system, 

rays from all zones are centered on the central 

ray, so stars appear round across the field. 

Field Curvature 

Field curvature occurs when the best off-axis 

images in an optical system focus ahead or 

behind the focused on-axis image. The result 

is that star images in the center of the field of 

view are sharp, but off-axis images appear more 

and more out of focus. A telescope with no field 

curvature has a “flat field,” so images are sharp 

across the whole field of view. 

Spherochromatism 

In the Schmidt Cassegrain, spherochromatism 

is present, but not deleterious in designs with 

modest apertures and focal ratios. It occurs 

because the optical “power” of the Schmidt 

corrector plate varies slightly with wavelength. 

Only in very large apertures or fast SCTs does 

spherochromism become a problem. 

For more than forty years, the classic SCT satisfied 

the needs of visual observers and astrophotographers. 

Its performance resulted from a blend of smooth spherical 

surfaces and Johnson’s unique method of producing 

the complex curve on the corrector with the same 

ease as producing spherical surfaces. As the 21st Century 

began, two emerging technologies—wide‐field 

eyepieces and CCD cameras—demanded high‐quality 

images over a much wider field of view than the classical 

SCT could provide. 

Why? The classic SCT is well corrected optically for 

aberrations on the optical axis, that is, in the exact center 

of the field of view. Away from the optical axis, 

however, its images suffer from two aberrations: coma 

and field curvature. Coma causes off‐axis star images to 

flare outward; field curvature causes images to become 

progressively out of focus away from the optical axis. 

As wide‐field eyepieces grew in popularity, and as 

observers equipped themselves with advanced CCD 

cameras, the classic SCT proved inadequate. To meet 

the requirements of observers, we wanted the new 

Celestron optics to be both free of coma and to have 

virtually zero field curvature. 

3. Engineering a New Astrograph 

We did not take lightly the task of improving the 

classic SCT. Its two spherical mirrors and our method 

of making corrector lenses allowed us to offer a highquality 

telescope at a low cost. We investigated the 

pros and cons of producing a Ritchey‐Chrétien (R‐C) 

Cassegrain, but the cost and complexity of producing 

its hyperbolic mirrors, as well as the long‐term disadvantages 

of an open‐tube telescope, dissuaded us. We 

also designed and produced two prototype Corrected 

Dall‐Kirkham (CDK) telescopes, but the design’s ellipsoidal 

primary mirror led inevitably to a more expensive 

instrument. While the R‐C and CDK are fine 

optical systems, we wanted to produce equally fine 

imaging telescopes at a more affordable price. 

As we’ve already noted, our most important design 

goal for the new telescope was to eliminate coma and 

field curvature over a field of view large enough to 

accommodate a top‐of‐the‐line full‐frame digital SLR 

camera or larger astronomical CCD camera. Translated 

to engineering requirements, this meant setting the 

field of view at 42 mm in diameter. And, of course, any 

design that would satisfy the full‐frame requirement 

would also be great for the less expensive APS‐C digital 

SLR cameras (under $800) and less expensive astronomical 

CCD cameras (under $2,000). 

There are several ways to modify the classic SCT to 

reduce or eliminate coma. Unfortunately, these methods 

leave uncorrected field curvature. We could 

replace either the spherical primary or secondary with 

an aspheric (i.e., non‐spherical) mirror. Making the 

smaller secondary mirror into a hyperboloid was an 

obvious choice. But although this would certainly have 


The Optical Performance of the EdgeHD Compared to Other SCTs 

Classical SCT 

“Coma-Free” SCT 

100 μm 

EdgeHD SCT 

On-Axis 5.00 mm 10.00 mm 15.00 mm 20.00 mm 

Off-axis distance (millimeters) 

Figure 2. Matrix spot diagrams compare the centerto-edge 

optical performance of the EdgeHD, a “comafree” 

SCT, and the classic SCT. The EdgeHD is clearly 

the winner. The classic SCT shows prominent coma. 

The “coma-free” SCT is indeed free of coma, but field 

curvature causes its off-axis images to become diffuse 

and out of focus. In comparison, the EdgeHD’s 

spot pattern is tight, concentrated, and remains small 

from on-axis to the edge of the field. 

given us a coma‐free design, its uncorrected field curvature 

leaves soft star images at the edges of the field. 

We were also concerned that by aspherizing the secondary, 

the resulting coma‐free telescopes would 

potentially have zones that would scatter light and 

compromise the high‐power definition that visual 

observers expect from an astronomical telescope. Furthermore, 

the aspheric secondary mirror places 

demands on alignment and centration that often result 

in difficulty maintaining collimation. 

The inspiration for the EdgeHD optics resulted from 

combining the best features of the CDK with the best 

features of the classical SCT. We placed two small 

lenses in the beam of light converging toward focus 

and re‐optimized the entire telescope for center‐toedge 

performance. In the EdgeHD, the primary and 

secondary mirrors retain smooth spherical surfaces, 

and the corrector plate remains unchanged. The two 

small lenses do the big job of correcting aberrations for 

a small increment in cost to the telescope buyer. Furthermore, 

because the EdgeHD retains key elements of 

the classic SCT, the EdgeHD design is compatible with 

the popular Starizona Hyperstar accessory. You simply 

remove the secondary mirror and insert the Hyperstar. 

4. Optical Performance of the EdgeHD 

Optical design involves complex trade‐offs between 

optical performance, mechanical tolerances, cost, manufacturability, 

and customer needs. In designing the 

EdgeHD, we placed optical performance first: the 

instrument would be diffraction limited on axis, it 


Field Curvature 

Telescope with Field Curvature 

Flat-Field Telescope 

Figure 3. In an optical system with field curvature, 

objects are not sharply focused on a flat surface. 

Instead, off-axis rays focus behind or ahead of the 

focus point of the on-axis rays at the center of the 

field. As a result, the off-axis star images are 

enlarged by being slightly out of focus. 

would be entirely coma‐free, and the field would be 

flat to the very edge. (Indeed, the name of the EdgeHD 

derives from our edge‐of‐field requirements.) 

Figure 2 shows ray‐traced spot diagrams for the 

classic Schmidt‐Cassegrain, a “coma‐free” SCT with an 

aspheric secondary mirror, and the coma‐free, flat‐field 

EdgeHD design. All three are 14‐inch aperture telescopes. 

We use ZEMAX® professional optical ray‐trace 

software to design the EdgeHDs and to produce these 

ray‐trace data for you. 

Each spot pattern combines spots at three wavelengths: 

red (0.656 μm), green (0.546 μm), and blue 

(0.486 μm) for five field positions: on‐axis, 5 mm, 

10 mm, 15 mm, and 20 mm off‐axis distance. The field 

of view portrayed has diameter of 40 mm—just a bit 

under the full 42 mm image circle of the EdgeHD—and 

the wavelengths span the range seen by the darkadapted 

human eye and the wavelengths most often 

used in deep‐sky astronomical imaging. 

In the matrix of spots, examine the left hand column. 

These are the on‐axis spots. The black circle in 

each one represents the diameter of the Airy disk. If the 

majority of the rays fall within the circle representing 

the Airy disk, a star image viewed at high power will 

be limited almost entirely by diffraction, and is therefore 

said to be diffraction limited. By this standard, all 

three SCT designs are diffraction limited on the optical 

axis. In each case, the Schmidt corrector removes 

spherical aberration for green light. Because the index 

of refraction of the glass used in the corrector plate varies 

with wavelength, the Schmidt corrector allows a 

small amount of spherical aberration to remain in red 

and blue light. This aberration is called spherochromatism, 

that is, spherical aberration resulting from the 

color of the light. While the green rays converge to a 

nearly perfect point, the red and blue spot patterns fill 

or slightly over‐fill the Airy disk. Numerically, the 

radius of the Airy disk is 7.2 μm, (14.4 μm diameter) 

while the root‐mean‐square radius of the spots at all 

three wavelengths is 5.3 μm (10.6 μm diameter). 

Because the human eye is considerably more sensitive 

to green than it is to red or blue, images in the eyepiece 

appear nearly perfect even to a skilled observer. 

Spherochromatism depends on the amount of correction, 

or the refractive strength, of the Schmidt lens. 

To minimize spherochromatism, high‐performance 

SCTs have traditionally been ƒ/10 or slower. When 

pushed to focal ratios faster than ƒ/10 (that is, when 

pushed to ƒ/8, ƒ/6, etc.) spherochromatism increases 

undesirably. 

Next, comparing the EdgeHD with the classic SCT 

and the “coma‐free” SCT, you can see that off‐axis 

images in the classic SCT images are strongly affected 

by coma. As expected, the images in the coma‐free 

design do not show the characteristic comatic flare, but 

off‐axis they do become quite enlarged. This is the 

result of field curvature. 

Figure 3 illustrates how field curvature affects offaxis 

images. In an imaging telescope, we expect on‐axis 

and off‐axis rays to focus on the flat surface of a CCD 

or digital SLR image sensor. But unfortunately, with 

field curvature, off‐axis rays come to sharp focus on a 

curved surface. In a “coma‐free” SCT, your off‐axis star 

images are in focus ahead of the CCD. 

At the edge of a 40 mm field, the “coma‐free” telescope’s 

stars have swelled to more than 100 μm in 

diameter. Edge‐of‐field star images appear large, soft, 

and out of focus. 

Meanwhile, at the edge of its 40 mm field, the 

EdgeHD’s images have enlarged only slightly, to a 

root‐mean‐square radius of 10.5 μm (21 μm diameter). 

But because the green rays are concentrated strongly 

toward the center, and because every ray, including the 

faint ʺwingsʺ of red light, lie inside a circle only 50 μm 

in diameter, the images in the EdgeHD have proven to 

be quite acceptable in the very corners of the image 

captured by a full‐frame digital SLR camera. 

Field curvature badly affects imaging when you 

want really good images across your field of view. The 

effects of field curvature are demonstrated clearly in 

Figure 4 (for 8‐inch telescopes) and Figure 5 (for 14‐ 

inch telescopes). Note how the spot patterns change 

with off‐axis distance and focus. A negative focus distance 

means closer to the telescope; a positive distance 

mean focusing outward. In the EdgeHD, the smallest 

spots all fall at the same focus position. If you focus on 

a star at the center of the field, stars across the entire 

field of view will be in focus. 

In comparison, the sharpest star images at the edge 

of the field in the “coma‐free” telescope come to focus 

in front of the on‐axis best focus. If you focus for the 


8” ƒ/10 Coma-Free SCT 

-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 

8” ƒ/10 Flat-Field EdgeHD 

-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 

On-axis 

3.5 mm 

off-axis 

7 mm 

off-axis 

10.5 mm 

off-axis 

14 mm 

off-axis 

Spot diagrams plotted for 0.0, 3.5, 7, 10.5, and 14 mm off axis; showing λ = 0.486, 0.546, and 0.656 μm. 

center of the image, star images become progressively 

enlarged at greater distances. The best you can do is 

focus at a compromise off‐axis distance, and accept 

that you’ll have slightly out‐of‐focus stars both on‐axis 

and at the edge of the field. 

Any optical designer possessing the requisite skills 

with access to a computer equipped with optical raytracing 

software can—in theory—replicate and verify 

the optical performance of EdgeHD optics, so you 

don’t need to take our word for it. If you make the comparison 

yourself, you see that eliminating coma alone 

is not enough to guarantee good images across the 

field of view. For high‐performance imaging, an imaging 

telescope must be diffraction limited on axis and 

corrected for both coma and field curvature off‐axis. 

And that’s what you get with the EdgeHD, and at a 

very affordable price. 

5. Mechanical Design Improvements 

To insure that the completed EdgeHD telescope 

delivers the full potential of the optical design, we also 

redesigned key mechanical components. With classic 

SCT designs, for example, an observer could bring the 

optical system to focus at different distances (that is, 

different back‐focus distances) behind the optical tube 

assembly. Doing so changes the effective focal length of 

Figure 4. Compare star images formed by a 8-inch 

coma-free SCT with those formed by an EdgeHD. The 

sharpest star images in the coma-free SCT follow the 

gray curve, coming to focus approximately 0.6 mm in 

front of the focal plane. In the EdgeHD, small, tight 

star images are focused at the focal plane across the 

field of view, meaning that your images will be crisp 

and sharp to the very edge. 

the telescope, causes on‐axis spherical aberration, and 

increases the off‐axis aberrations. In the EdgeHD 

series, the back focus distance is optimized and set for 

one specific distance. Every EdgeHD comes equipped 

with a visual back that places the eyepiece at the correct 

back‐focus distance, and our Large T‐Adapter 

accessory automatically places digital SLR cameras at 

the optimum back‐focus position. 

As part of the optical re‐design, we placed the primary 

and secondary mirrors closer than they had been 

in the classic SCT, and designed new baffle tubes for 

both mirrors that allow a larger fully‐illuminated field 

of view. To insure full compatibility with the remarkable 

Starizona Hyperstar accessory that enables imaging 

at ƒ/1.9 in the EdgeHD 800 and ƒ/2.0 in the EdgeHD 

925, 1100, and 1400, all EdgeHDs have a removable secondary 

mirror. 


14” ƒ/10 Coma-Free SCT 

-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 

14” ƒ/11 Flat-Field EdgeHD 

-0.8 mm -0.4 mm 0.0 mm +0.4 mm +0.8 mm 

On-axis 

5 mm 

off-axis 

10 mm 

off-axis 

15 mm 

off-axis 

20 mm 

off-axis 

Spot diagrams plotted for 0.0, 5, 10, 15, and 20 mm off axis; showing λ = 0.486, 0.546, and 0.656 μm. 

Figure 5. In a 14-inch coma-free SCT, the smallest 

off-axis star images lie on the curved focal surface 

indicated by the gray line. Since CCD or digital SLR 

camera is flat, so star images at the edge of the field 

will be enlarged. In the aplanatic EdgeHD design, the 

smallest off-axis images lie on a flat surface. Stars 

are small and sharp to the edge of the field. 

Because it covers a wide field of view, the optical 

elements of the EdgeHD must meet centering and 

alignment tolerances considerably tighter than those of 

the classic SCT design. For example, because the corrector 

plate must remain precisely centered, we secure 

it in place with alignment screws tipped with soft 

Nylon plastic. The screws are set on the optical bench 

during assembly while we center the corrector plate. 

Once this adjustment is perfect, the screws are tightened 

and sealed with Loctite® to maintain the corrector 

in position. This seemingly small mechanical 

change ensures that the corrector plate and the secondary 

mirror mounted on the corrector plate stay in permanent 

optical alignment. 

Centering the primary is even more demanding. In 

the classic SCT, the primary mirror is attached to a sliding 

“focus” tube. When you focus the telescope, the 

focus knob moves the primary mirror longitudinally. 

When you reverse the direction of focus travel, the 

focus tube that carries the primary can ʺrockʺ slightly 

on the baffle tube, causing the image to shift. In the 

classic SCT, the shift does not significantly affect onaxis 

image quality. However, in the EdgeHD, off‐axis 

images could be affected. Because the baffle tube carries 

the sub‐aperture corrector inside and the primary 

mirror on the outside, we manufacture it to an 

extremely tight diametric tolerance. The tube that supports 

the primary was re‐designed with a centering 

and alignment flange which contacts the optical (front) 

surface of the primary mirror. When the primary mirror 

is assembled onto the focus tube and secured with 

RTV adhesive, this small mechanical change guarantees 

precise optical centration. Following assembly, the 

focus tube carrying the primary is placed in a test jig. 

We rotate the mirror and verify that the primary is precisely 

squared‐on to insure that the full image quality 

expected from the optics is maintained. 

In any optical system with a moveable primary mirror, 

focus shift—movement of the image when the 

observer changes focusing direction—has been an 

annoyance. In Celestron’s SCT and EdgeHD telescopes, 

we tightened the tolerances. During assembly and testing, 

we measure the focus shift; any unit with more 

than 30 arcseconds focus shift is rejected and returned 

to an earlier stage of assembly for rework. 

In the classic SCT, astrophotographers sometimes 


experienced an image shift as the telescope tracked 

across the meridian. The focus mechanism serves as 

one support point for the mirror. In the EdgeHD, we 

added two stainless steel rods to the back of the cell 

that supports the primary mirror. When the two mirror 

clutches at the back of the optical tube assembly are 

engaged, aluminum pins press against the stainless 

steel rods, creating two additional stabilizing support 

points (see Figure 6). 

Telescope tubes must ʺbreatheʺ not only to enable 

cooling, but also to prevent the build‐up of moisture 

and possible condensation inside the tube. In the classic 

SCT, air can enter through the open baffle tube. In 

the EdgeHD, the sub‐aperture lenses effectively close 

the tube. To promote air exchange, we added ventilation 

ports with 60‐μm stainless steel mesh that keeps 

out dust but allows the free passage of air. 

Observers expect—in a telescope designed for imaging—to 

attach heavy filter wheels, digital SLRs, and 

astronomical CCD cameras. We designed the rear 

threads of the EdgeHD 925, 1100, and 1400 telescopes 

with a heavy‐duty 3.290×16 tpi thread, and we set the 

back focus distance to a generous 5.75 inches from the 

flat rear surface of the baffle tube locking nut. The rear 

thread on the EdgeHD 800 remains the standard 

2.00×24 tpi, and the back‐focus distance is 5.25 inches. 

Many suppliers offer precision focusers, rotators, filter‐wheels, 

and camera packages that are fully compatible 

with the heavy‐duty rear thread and back‐focus 

distance of the EdgeHD. 

6. Manufacturing the EdgeHD Optics 

Each EdgeHD has five optical elements: an aspheric 

Schmidt corrector plate, a spherical primary mirror, a 

spherical secondary mirror, and two sub‐aperture corrector 

lenses. Each element is manufactured to meet 

tight tolerances demanded by a high‐performance 

Figure 6. The mirror clutch mechanism shown in this 

cross-section prevents the primary mirror from shifting 

during the long exposures used in imaging. 

Figure 7. Matchplates use interference fringes to 

check the radius and smoothness of the correction. In 

this picture, you see a corrector blank attached to a 

master block. The matchplate rests on top; interference 

fringes appear as green and blue circles. The 

circular pattern indicates a difference in radius. 

optical design. Celestron applies more than forty years 

of experience in shaping, polishing, and testing astronomical 

telescope optics to each and every one of the 

components in each EdgeHD telescope. Our tight specs 

and repeated, careful testing guarantee that the telescope 

will not only perform well for high‐power planetary 

viewing, but will also cover a wide‐angle field for 

superlative edge‐to‐edge imaging. Nevertheless, we 

donʹt take this on faith; both before and after assembly, 

we test and tune each set of optics. 

Celestronʹs founder, Tom Johnson, invented the 

breakthrough process used to make Celestronʹs corrector 

plates. Over the years, his original process has been 

further developed and refined until, at present, we 

manufacture corrector plates with the same level of 

ease, certainty, and repeatability that opticians expect 

when they are producing spherical surfaces. 

Each corrector plate begins life as a sheet of waterwhite, 

high transmission, low‐iron, soda‐lime float 

glass. In manufacturing float glass, molten glass is 

extruded onto a tank of molten tin, where the glass 

floats on the dense molten metal. The molten tin surface 

is very nearly flat (its radius of curvature is the 

radius of planet Earth!), and float glass is equally flat. 

We cut corrector blanks from large sheets of the glass, 

then run them through a double‐sided surfacing 

machine to grind and polish both surfaces to an optical 

finish. The blanks are inspected and any with defects 

are discarded. 

The Johnson/Celestron method for producing the 

polynomial aspheric curve is based on precision ʺmaster 

blocksʺ with the exact inverse of the desired curve. 

We clean the master block and corrector blank, and 

then, by applying a vacuum from the center of the 

block, pull them into intimate optical contact, exclud‐ 


Clean Separation 

Round Stars... 

...at the very 

edge of the field. 

Amazing 

Features 

Round Stars 

in the Center 

Extraordinary 

Detail! 

EdgeHD’s Close-Up 

on the Pelican Nebula 

14” ƒ/10.8 Celestron EdgeHD telescope 

Image scale: 0.484 arcseconds per pixel 

Image size: 21.5×29.8 mm field of view 

Field of view: 27.3×21.5 arc-minutes 

Figure 8. After all the testing is done, the ultimate 

test is the night sky. This close-up image of the Pelican 

Nebula testifies to the EdgeHD’s ability to focus 

clean, neat, round star images from center to edge. 

Image by André Paquette 

The telescope was a 14-inch EdgeHD on a CGE Pro 

mounting; the CCD camera an Apogee U16m. The 

image above shows a 21.5 × 29.8 mm section 

cropped from the original 36.8 mm square image. 


ing any lint, dust, or air between them, gently bending 

the flat corrector blank to match the reverse curve of 

the block. We then take the combined master block and 

corrector blank and process the top surface of the corrector 

to a polished concave spherical surface. With the 

corrector lens still on the master block, an optician tests 

the radius and figure of the new surface against a precision 

reference matchplate (also known as an optical 

test plate or test glass) using optical interference to read 

the Newtonʹs rings or interference fringes, as shown in 

Figure 7. If the surface radius lies within a tolerance of 

zero to three fringes (about 1.5 wavelengths of light, or 

750 nm concave), and the surface irregularity is less 

than half of one fringe (¼–wavelength of light), the corrector 

is separated from the master block. The thin 

glass springs back to its original shape, so that the side 

that was against the master block becomes flat and the 

polished surface assumes the profile of a Schmidt corrector 

lens. The corrector is tested again, this time in a 

double‐pass auto collimator. Laser light at 532 nm 

wavelength (green) enters through an eyepiece, strikes 

an EdgeHD secondary and primary mirror, passes 

through the corrector lens under test, reflects from a 

precision optical flat, then goes back through the corrector 

to reflect again from the mirrors, and finally 

back to focus. Because the light passes twice through 

the Schmidt corrector lens, any errors are seen doubled! 

The double‐pass autocollimation test (see Figure 

9) insures that every Schmidt corrector meets the stringent 

requirements of an EdgeHD optical system. 

Primary mirrors begin as precision‐annealed 

molded castings of low‐expansion borosilicate glass 

with a weight‐saving conical back surface and a concave 

front surface. The molded casting is edged round, 

its central hole is cored, and the radius of the front surface 

is roughed in. Celestron grinds the front surface of 

primary mirrors with a succession of progressively 

Autocollimation Testing 

Telescope being checked 

Precision optical flat 

Beamslitter 

Eyepiece and Ronchi grating 

Green Laser (532 nm) 

Figure 9. In autocollimation testing, light goes 

through an optical system, reflects from a plane mirror, 

and passes through again. This super-sensitive 

test method doubles the apparent size of all errors. 

Figure 10. We test all of our primary mirrors on an 

optical bench by means of laser interferometry. In the 

picture, stacks of polished primary mirrors await testing 

on one of our optical test benches. 

finer diamond abrasive pellet tools using high‐speed 

spindle machines, then transfers them to an abrasivefree 

room where they are polished to a precise spherical 

surface. Each mirror is checked for both radius and 

optical spherical figure against a convex precision reference 

matchplate. When the interference fringes indicate 

the radius is within ±1 fringe from the nominal 

radius and the surface irregularity is less than onefourth 

of one fringe, the mirror receives a final check 

using the classic mirror‐makerʹs null test familiar to 

every professional optican as well as every amateur 

telescope maker. Afterwards, every primary mirror is 

taken to the QA Interferometry Lab—shown in Figure 

10—where the surface irregularity of each mirror is 

verified, via interferometer, to be within specification. 

The smaller secondary mirrors are also made of 

low‐expansion borosilicate glass. Like the primaries, 

the secondaries are edged and centered, then ground 

and polished. The secondary is a convex mirror so during 

manufacture it is tested against a concave precision 

reference matchplate to check both its radius of curvature 

and figure. The secondary mirrors are also 

brought to the QA Interferometry Lab where the 

radius and irregularity of each mirror is verified 

through interferometric measurement to assure that 

each one lies within specification. 

When we designed the EdgeHD optical system, we 

strongly favored spherical surfaces because a sphere 

can be tested by optical interference to high accuracy in 

just a matter of minutes. If we had specified a hyperboloidal 

surface for the secondary mirror, we would have 

been forced to use slower, less accurate testing methods 

that might miss zonal errors. Furthermore, comafree 

SCT designs with hyperboloidal mirrors still suffer 

from field curvature—an aberration that we specifically 

wished to avoid in the EdgeHD design. 


Figure 11. To correct any remaining optical errors, 

the figure of the secondary mirror is fine-tuned 

against the entire optical system in double-pass autocollimation 

setup. This delicate match process insures 

that every telescope performs to the diffraction limit. 

Finally, the sub‐aperture corrector lenses are made 

using the same manufacturing techniques used with 

high‐performance refractor objectives. The EdgeHD 

design specifies optical glass from Schott AG. The 8‐, 

9.25‐, and 11‐inch use N‐SK2 and K10 glasses, while 

the 14‐inch uses N‐SK2 and N‐BALF2 glasses. To 

insure homogeneity, optical glass is made in relatively 

small batches, extruded in boules. The raw glass is then 

diamond milled to the correct diameter, thickness, and 

radii. Each lens blank is blocked, ground, and polished, 

then tested using matchplates to insure that the radius 

and figure meet the tight tolerances required of the 

EdgeHD sub‐aperture corrector lenses. 

Our assembly workstations resemble the optical 

benches used to qualify corrector plates. The primary 

mirror and corrector plate slip into kinematic support 

jigs, and we place the secondary mirror in its holder. 

The sub‐aperture corrector lenses meet specifications 

so reliably that a master set is used in the assembly 

workstation. Laser light from the focus position passes 

in reverse through the optics, reflects from a master 

autocollimation flat, then passes back through the 

optics. Tested in autocollimation, the optician can see 

and correct surface errors considerably smaller than a 

millionth of an inch. 

If the combined optics set shows any slight residual 

under‐ or over correction, zones, astigmatism, upturned 

or down‐turned edges, holes, or bulges, the 

optician marks the Foucault test shadow transitions on 

the secondary mirror, then removes the secondary mirror 

from the test fixture and translates these markings 

into a paper pattern. The pattern is pressed against a 

pitch polishing tool, and the optician applies corrective 

polishing to the secondary mirror—as we show in Figure 

11—until the optical system as a whole displays a 

perfectly uniform illumination (no unwanted zones or 

shadows) under the double‐pass Foucault test and 

smooth and straight fringes under the double‐pass 

Ronchi test. The in‐focus Airy disk pattern is evaluated 

for roundness, a single uniform diffraction ring, and 

freedom from scattered light. In addition, the intraand 

extra‐focal diffraction pattern must display the 

same structure and central obscuration on both sides of 

focus, and it must appear round and uniform. 

After we remove each set of optics from the autocollimator, 

we send the components to our in‐house coating 

chamber. Here, the primary and secondary mirrors 

receive their high‐reflectance aluminum coatings, and 

the corrector lens is anti‐reflectance coated. Each set of 

optics is then installed into an optical tube assembly 

(OTA). 

Completed OTAs now undergo the Visual Acceptance 

Test. In a temperature‐stabilized optical test tunnel, 

laser light at 532 nm wavelength (green) is 

reflected from a precision paraboloidal mirror to act as 

an artificial star. With a high‐power ocular, a QA 

Inspector views the artificial star critically. 

To pass, an OTA must meet these tough criteria: 

• The in‐focus Airy disk must be round, display only 

one bright ring, and it must be free of scattered 

light around the disk; 

• Inside and outside focus, the diffraction patterns 

must be round, uniform, and appear similar on 

both sides of focus; and 

• Observed with a 150 line‐pairs‐per‐inch Ronchi 

grating, the bands must be straight, uniformly 

spaced, and high in contrast. 

Because its optics have been tested and tuned in 

error‐revealing double‐pass mode, and because each 

assembled OTA has been tested again and qualified 

visually, when you observe the sky, your telescope’s 

images should be flawless. 

7. Final Acceptance Testing and Certification 

Before it can leave Celestron’s facilities, every 

EdgeHD must pass its Final Acceptance Test, or FAT. 

We conduct the FAT test on an optical test bench in a 

specially constructed temperature‐controlled room 

(Figure 12). Rather than use laser light for this test, we 

use white light so that the FAT reproduces the same 

conditions an observer would experience while viewing 

or photographing the night sky. To avoid placing 

any heat sources in the optical path, the light for our 

artificial star is carried to the focus of a precision parabolic 

mirror through a fiber‐optic cable. After striking 

the parabolic mirror, the parallel rays of light travel 

down the optical bench to the EdgeHD under test, 

through the telescope, to a full‐frame format digital 

SLR camera placed at its focus. 

Using a set of kinematic test cradles, there is no need 

to change the test configuration between different 


Figure 12. In the final acceptance test, or FAT, for an 

EdgeHD, the optics must demonstrate the ability to 

form sharp images at the center and in the corners of 

a Canon 5D Mark II full-frame digital SLR camera, 

with a sensor that measures 42 mm corner-to-corner. 

EdgeHD telescopes. We simply place the telescope in 

its test cradle on the bench, and itʹs ready for testing. 

The Final Acceptance Test verifies an EdgeHD’s ability 

to form sharp star images in the center and to the 

edges of a full‐frame (24×36 mm format, with a 42 mm 

diagonal measurement) digital SLR camera. The QA 

Inspector attaches a full‐frame digital SLR camera to 

the telescope, focuses carefully, and takes an on‐axis 

image. The telescope is then pointed so the artificial 

star image falls in the corner of the frame, and without 

refocusing, the inspector takes another image. The process 

is repeated for each corner of the camera frame, 

and another picture is taken at the center of the frame. 

To pass the test, the telescope must form a sharp 

image at the center of the field, at each corner of the 

camera frame, and again at the center. The images are 

examined critically. To pass, every one of the test 

images must be tight, round, and in perfect focus. Any 

EdgeHD that does not pass the FAT is automatically 

returned to the assembly room to recheck the collimation 

and centering of its corrector plate. No EdgeHD 

can leave the factory until it has passed its FAT. 

Throughout the telescope‐building process, we 

maintain a quality‐assurance paper trail for each 

instrument. All test images are numbered and cross 

referenced. Should a telescope be returned to Celestron 

for service, we can consult our records to see how well 

it performed before it left our facility. Once a telescope 

has passed the final acceptance test, we apply Loctite to 

the set screws to permanently hold the alignment of 

the corrector plate. The instrument is then inspected 

carefully for cosmetic defects. It is cleaned and packaged 

for shipment to our dealers and customers. 

8. Visual Observing with the EdgeHD 

Because both the Celestron EdgeHD and our classical 

SCTs are diffraction‐limited on axis, their performance 

is essentially the same for high‐magnification 

planetary, lunar viewing, splitting close double stars, 

or any other visual observing task that requires firstrate 

on‐axis image quality. However, the EdgeHD outshines 

the classic SCT when it comes to observing 

deep‐sky objects with the new generation of high‐performance 

wide‐field eyepieces. 

The classic SCT exhibits off‐axis coma and field curvature 

which are absent from the EdgeHD design. 

Modern wide‐field eyepieces, such as the 23 mm Luminos, 

have an apparent field of view of 82 degrees, so 

they show you more sky. And gone are the light‐robbing 

radial flares of coma and annoying, out‐of‐focus 

peripheral images so sadly familiar to observers. With 

the EdgeHD, stars are crisp and sharp to the edge. 

The back of the EdgeHD 800 features an industry 

standard 2.00×24 tpi threaded flange. A large retaining 

ring firmly attaches the 1¼‐inch visual back, and this 

accepts a 1¼‐inch Star Diagonal that will accept any 

standard 1¼‐inch eyepiece. 

The EdgeHD 925, 1100, and 1400 feature a heavyduty 

flange with a 3.290×16 tpi threaded flange. This 

oversize flange allows you to attach heavy CCD cameras 

and digital SLR cameras. For visual observing, use 

the adapter plate supplied with each telescope to 

attach the Visual Back. The 2‐inch XLT Diagonal (also 

supplied with these telescopes) accepts eyepieces with 

1¼‐inch and 2‐inch barrels. 

To your discerning eye—as an observer with experi‐ 

APS-C DSLR 

EdgeHD Field of View 

42 mm ∅ 

KAF-3200 

Full-Frame DSLR 

KAI-10002 

KAF-8300 

KAF-16803 

Figure 13. The EdgeHD telescopes are designed to 

provide good images across a flat 42 mm diameter 

field of view. Compare this with the size of a variety 

of image sensor formats. The popular APS-C digital 

SLR format fits easily. The full-frame DSLR format is 

fully covered. But the EdgeHDs cover even the 

36.8 mm square KAF-16803 format remarkably well. 


Imaging with Celestron EdgeHD Telescopes 

EdgeHD 

Aperture 

Focal Ratio 

Focal Length 

Secondary Ø 

Obscuration 1 

Back Focus 

Distance 

Adapter 

Thread Size 

Image Circle 

Linear Ø 

Angular Ø 

Airy Disk 

Angular Ø 

Linear Ø 

Rayleigh 2 

Image Scale 

arcsec/pixel 

(6.4 μm 

pixel) 


800 

203.2 mm 

ƒ/10.456 

2125 mm 

68.6 mm 

34% 

133.35 mm 

2.00”-24 tpi 

42 mm Ø 

68.0 arcmin 

1.36” Ø 

14.0 μm Ø 

0.68” 

0.62”/pix 


925 

235 mm 

ƒ/9.878 

2321 mm 

85.1 mm 

36% 

146.05 mm 

3.29”-16 tpi 

42 mm Ø 

62.2 arcmin 

1.18” Ø 

13.2 μm Ø 

0.59” 

0.57”/pix 


1100 

279.4 mm 

ƒ/9.978 

2788 mm 

92.3 mm 

33% 

146.05 mm 

3.29”-16 tpi 

42 mm Ø 

51.8 arcmin 

0.99” Ø 

13.3 μm Ø 

0.50” 

0.47”/pix 


1400 

355.6 mm 

ƒ/10.846 

3857 mm 

114.3 mm 

32% 

146.05 mm 

3.29”-16 tpi 

42 mm Ø 

37.4 arcmin 

0.78” Ø 

14.4 μm Ø 

0.39” 

0.34”/pix 

1. The Ø symbol means diameter. Central obscuration is given as a percentage of the aperture. 

2. The Rayleigh Limit for resolving doubles with equally bright components. The” symbol means arcseconds. 

ence—on a night with steady air and good seeing, a 

properly cooled EdgeHD performs exceptionally well 

on stars. You will see a round, clean Airy disk, a single 

well‐defined diffraction ring, and symmetrical images 

inside and outside of focus. Every EdgeHD should 

resolve double stars to the Dawes limit, reveal subtle 

shadings in the belts of Jupiter, and show easily the 

Cassini Division in Saturnʹs rings. On deep‐sky objects 

viewed with a high‐quality eyepiece, star images 

appear sharp and well defined to the edge of the field 

of view, and to your dark‐adapted eyes, the EdgeHD 

reveals faint nebular details as fine as the sky quality at 

the observing site will allow. 

9. Imaging with the EdgeHD 

The Celestron EdgeHD was designed and optimized 

for imaging with astronomical CCD cameras, 

digital SLR cameras, video astronomy sensors, electronic 

eyepieces, and webcams. We designed the 

EdgeHD 800 to deliver the best images 5.25 inches 

(133.35 mm) behind the surface of the telescope’s rear 

cell 2.00×24 tpi threaded baffle tube lock nut, and the 

EdgeHD 925, 1100, and 1400 form their best images 

5.75 inches (146.05 mm) behind the telescope’s rear cell 

3.290×16 tpi threaded baffle tube lock nut. For best 

results, the image sensor should be located within 

±0.5 mm of this back‐focus distance. 

It is easy to place a digital SLR (DSLR) camera at the 

proper distance using the small T‐Adapter (item 

#93644) for the EdgeHD 800, or the large T‐Adapter 

(item #93646) for the EdgeHD 925, 1100, and 1400. The 

small adapter is 78.35 mm long while the large adapter 

adds 91.05 mm, in both cases placing the best focus 

55 mm behind the T‐Adapter. Because 55 mm is the 

industry standard T‐mount to sensor distance, add a T‐ 

Ring adapter (T‐Ring for Canon EOS, item #93419; T‐ 

Ring for Nikon, item #93402) and attach your camera to 

it. That’s all there is to placing your digital SLR camera 

at the correct back‐focus location. 

By the way, if you’ve never heard of the T‐mount 

system, you need to know about it. The T‐mount is a 

set of industry standard sizes and distances for camera 

lenses. A standard T‐mount thread (M42×0.75) is available 

for most astronomical CCD cameras. The standard 

T‐mount flange‐to‐sensor distance is 55 mm. 

The T‐mount system also makes spacing an astronomical 

CCD camera easy. Consult your CCD camera’s 

documentation to find the flange‐to‐sensor distance for 

your CCD camera. Attaching the Celestron T‐Adapter 

to your EdgeHD gives you the standard 55 mm spacing. 

If your CCD’s front flange‐to‐sensor distance is 

35 mm, you need an additional 20 mm distance. Order 

a 20 mm T‐mount Extension Tube (available from 

astronomy retailers) to get the correct back‐focus distance. 

If you require a more complex optical train for 

your CCD camera, check the imaging accessories 

offered by astronomy retailers. 

For imaging, we recommend using T‐system components 

because threaded connections place your CCD 

camera or digital SLR at the correct back focus distance 

for optimum performance. Not only are they strong, 

but they also hold your camera perfectly square to the 

light path. 

To mount a high‐performance video camera, add 


Celestron’s EdgeHD: The Versatile Imaging Platform 


800 

5.25 inches 

133.35±0.5 mm 

Small T-Adapter 

Camera Adapter 

Digital SLR 

Reducing Ring 

Small T-Adapter 

T-to-1.25” Adapter 

Webcam 

Large T-Adapter 

T-Ring Adapter 

Digital SLR 


925, 1100, 

and 1400 

5.75 inches 

146.05±0.5 mm 


T-to-C Adapter 

Astro Video Camera 


T-system Spacer 

CCD Camera 

the T‐Adapter plus a T‐to‐C adapter. (Like the T‐mount 

system, the C‐mount system is a industry standard. It 

uses a 1×32 tpi threads with a back‐focus distance of 

17.5 mm.) Almost all industrial‐grade astronomical 

video cameras use the C‐mount system. 

For consumer video systems such as electronic eyepieces, 

planetary cameras, and webcams that attach to 

the telescope using a standard 1.25‐inch eyepiece barrel, 

simply use the same components that you use for 

visual observing. Just remove the eyepiece from the 

telescope and replace it with the camera. 

For many imaging programs, you can simply shoot 

short exposures through the telescope. On a solid, 

polar‐aligned equatorial mounting, you may be able to 

Figure 14. It is easy to position your digital SLR camera, 

the astronomical CCD camera of your dreams, as 

well as high-performance video and inexpensive webcams 

at the focus plane of your EdgeHD telescope. 

For the sharpest wide-field imaging, your goal is to 

place the sensor 5.25 inches behind the rear flange of 

the EdgeHD 800, or 5.75 inches behind the EdgeHD 

925, 1100, and 1400 rear flange. 

expose for 30 seconds or more. With such exposure 

times, you can capture wonderful images of the moon, 

planets, eclipses, bright star clusters, and objects like 

the Orion Nebula. 

However, for long exposures on deep‐sky objects, 


you will need to guide the telescope. The days of guiding 

by eye are now long gone: electronic auto‐guiders 

are the best way to go. A functional and relatively inexpensive 

autoguiding setup consists of a small refractor 

mounted piggyback of your EdgeHD telescope. You 

will need a dovetail bar attached to the EdgeHD tube. 

Celestron offers an 80 mm guide telescope package 

(item #52309) to be used with the NexGuide Autoguider 

(item #93713). For sub‐exposures exceeding 10 

minutes or so, piggybacked guide telescopes potentially 

suffer from differential flexure; for such imaging, 

consider an off‐axis guiding system. 

For those who wish to make images with a faster 

focal ratio than EdgeHD 1100’s ƒ/10 or the EdgeHD 

1400’s ƒ/11, we designed a five‐element 0.7× reducer 

lens for each of these EdgeHD telescopes. (For more 

information, see Appendix B.) The Reducer Lens 0.7× 

for the EdgeHD 1100 is item #94241; for the 14‐inch, 

item #94240. (As of this writing, a focal reducer for the 

EdgeHD 800 is under development.) 

The reducer lens attaches directly to the 3.290×16 tpi 

threaded baffle tube lock nut on the back of the telescope. 

Since the back focus distance for the reducer 

lens is 5.75 inches (146.05 mm), you can use the same T‐ 

Adapter and camera T‐Ring you would use for imaging 

at the ƒ/10 or ƒ/11 focus. The linear field of view is 

still 42 mm diameter, but the angular field is 43% 

larger, and exposure times drop by a factor of two. 

For super‐fast, super‐wide imaging, the EdgeHD 

telescope series supports Starizona’s Hyperstar lens. 

Mounted on the corrector plate in place of the secondary 

mirror, the Hyperstar provides an ƒ/1.9 focal ratio 

on the EdgeHD 1400, and ƒ/2.0 or ƒ/2.1 on the 800, 925, 

and 1100. Covering a 27 mm diameter field of view, the 

Hyperstar is a perfect match for APS‐C format digital 

SLR cameras. Because of the short focal length and fast 

focal ratio, sub‐exposures are just a few minutes, and 

with a solid, polar‐aligned equatorial mount, guiding 

is seldom necessary. 

Of course, the focal length of any EdgeHD telescope 

can be extended with a Barlow lens (such as the 

Celestron 2× X‐Cel LX (item #93529) or 3× X‐Cel LX 

(item #93428)) into the desirable ƒ/22 to ƒ/32 range for 

ultra‐high‐resolution lunar and planetary imaging. 

In summary, the Celestron EdgeHD telescopes provide 

a flexible platform for imaging. You can work at 

the normal ƒ/10 or ƒ/11 Cassegrain focus for seeinglimited 

deep‐sky images or add the reducer lens for 

wider fields and shorter exposure times. With a Hyperstar, 

you can grab wide‐field, deep‐sky images in mere 

minutes. And finally, you can extend the focus to capture 

fine lunar and planetary images with a quality 

Barlow lens. When you buy an EdgeHD telescope, 

you’re getting an imaging platform that covers all the 

bases, from fast, wide‐field imaging to high‐resolution 

imaging of the moon and planets. 

10. Conclusion 

The classic Schmidt‐Cassegrain telescope introduced 

tens of thousands of observers and imagers to 

astronomy and nurtured the appreciation for the wonder 

of the night sky. But today, observers and imagers 

want a more capable telescope, a telescope that provides 

sharp close‐ups as well as high‐quality images all 

the way across a wide, flat field of view. And they want 

that telescope at an affordable price. At Celestron we 

designed the EdgeHD to satisfy these needs. The 

EdgeHD is not only coma‐free, but it also provides a 

flat field so that stars are sharp to the very edge of the 

field of view. In this brief technical white paper, we 

have shown you the inner workings of our new design, 

and demonstrated the care we exert as we build and 

test them. We trust that we have proven that an 

EdgeHD is the right telescope for you. 

11. References 

The reader may find the following resources to be 

useful in learning about optical design, fabrication, and 

testing: 

DeVany, Arthur S., Master Optical Techniques. John 

Wiley and Sons, New York, 1981. 

Fischer, Robert E.; Biljana Tadic‐Galeb; and Paul R. 

Yoder, Optical System Design. McGraw Hill, New 

York, 2008. 

Geary, Joseph M., Introduction to Lens Design. Willmann‐Bell, 

Richmond, 2002. 

Malacara, Daniel, ed., Optical Shop Testing. John Wiley 

and Sons, New York, 1978. 

Rutten, Harrie, and Martin van Venrooij, Telescope 

Optics: A Comprehensive Manual for Amateur Astronomers. 

Willmann‐Bell, Richmond, 1999. 

Smith, Gregory Hallock, Practical Computer‐Aided Lens 

Design. Willmann‐Bell, Richmond, 1998. 

Smith, Gregory Hallock; Roger Ceragioli; Richard 

Berry, Telescopes, Eyepieces, and Astrographs: Design, 

Analysis, and Performance of Modern Astronomical 

Optics. Willmann‐Bell, Richmond, 2012. 

Wikipedia. Search references to specific topics. See: 

http://en.wikipedia.org/wiki/Optical_lens_design 

and many associated links. 

Wikipedia. Search references to T‐mount. See: 

http://en.wikipedia.org/wiki/T‐mount 

and associated camera system links. 

Wilson, R. N., Reflecting Telescope Optics I and II. 

Springer‐Verlag, Berlin, 1996. 

ZEMAX® Optical Design Program, User’s Guide. Radiant 

Zemax LLC, Tucson, 2012. 


Appendix A: 

Technical Profiles of EdgeHD 

Telescopes 

Image quality in astronomical telescopes is determined 

by numerous factors that amateur astro‐imagers 

must bear in mind when evaluating their results. The 

major factors in play are: 

• the image formed by the telescope; 

• the sampling by pixels of the image sensor; 

• the diffraction pattern of the telescope; 

• the “seeing” quality during exposure; and 

• the guiding accuracy during exposure. 

To aid astro‐imagers, this Appendix presents a spot 

matrix plot for each of the telescopes in the EdgeHD 

series. To determine the size of the images that you 

observe in your exposures, these must be compounded, 

or convolved, with the other factors that 

affect your images. 

In the spot matrix plots we have provided, each 

large gray box is 64 μm on a side, and consists of a ten 

small boxes 6.4 μm representing a pixel in a “typical” 

modern CCD camera. The black circle represents the 

diameter of the Airy disk to the first dark ring. It is 

immediately clear that for each of the EdgeHDs, two 

6.4 μm pixels roughly match the diameter of the Airy 

disk. This means that under ideal conditions, a CCD 

camera with pixels of this size will capture most of the 

detail present in the telescopic image. Referring to the 

Figure A1, the left column shows the Airy disk for a 

telescope with a central obscuration of 34%. Because 

the light in the Airy disk is concentrated into a smaller 

area in the center, capturing all of the image detail in a 

planetary or lunar image requires using a 2x or 3x Barlow 

lens to further enlarge the Airy disk. 

Unfortunately, ideal conditions are fleeting. During 

a typical CCD exposure, atmospheric turbulence 

enlarges the image of all stars, and furthermore, it 

causes the images to wander. On the steadiest nights, 

the “seeing” effect may be as small as 1 second of arc. 

In Figure A1, the “superb seeing” column shows blurs 

14-inch 11-inch 9.25-inch 8-inch 

Airy Disk and Seeing Blurs 

Airy 

Disk 

Superb 

Seeing 

Excellent 

Seeing 

Average 

Seeing 

Figure A1. Shown at the same scale as the matrix 

spot diagrams are the Airy disk and the point-spreadfunction 

of seeing disks for average (2.0”), excellent 

(1.5”), and superb (1.0”) seeing. 

with a FWHM (full‐width half‐maximum) of 1 arcsecond. 

The next column shows excellent seeing (1.5”), 

and the right column shows 2” seeing blurs, typical of 

many nights at most observing sites. It is important 

note that as the focal length of the telescope increases, 

the diameter of the seeing blur increases in proportion. 

With a small telescope, seeing plays a small role. With 

the large‐apertures and long focal lengths of the 

EdgeHD series, nights of good seeing become particularly 

valuable. 


Celestron EdgeHD 800 

On‐axis, the spots show that the 8‐inch EdgeHD 

is diffraction limited in both green (for visual 

observing) and red (for imaging). And because 

blue rays are strongly concentrated inside the Airy 

disk, the 8‐inch EdgeHD is diffraction‐limited in 

blue light. Off‐axis, its images remain diffractionlimited 

over a field larger than the Full Moon. 

For an imager using an APS‐C digital SLR camera, 

relative illumination falls to 84% at the 

extreme corners of the image. Although for bright 

subjects this minor falloff would pass unnoticed, 

for imaging faint objects we recommend making 

and applying flat‐field images for the best results. 

For CCD imaging, we always recommend making 

flat field images. 

Portability and its affordable price are the hallmarks 

of the EdgeHD 800. Although the 8‐inch 

covers a 42 mm image circle, we optimized its 

optics for the central 28 mm area, the size of an 

APS‐C chip in many popular digital SLR cameras. 



The spot matrix shows that on‐axis images are 

diffraction limited at all three wavelengths, and 

remain diffraction limited over the central 15 mm. 

While blue and red are slight enlarged, in green 

light, however, images are fully diffraction‐limited 

over a full 38 mm image circle. The size of the offaxis 

blue and red spots are seen to remain nicely 

balanced. 

On a night of average seeing, stars will display a 

FWHM of 23μm, comparable in size to the spot 

pattern at the very edge of a 42 mm field. 

Relative illumination in the EdgeHD 925 is 

excellent. The central 12 mm is completely free of 

vignetting, while field edges receive fully 90% relative 

illumination. For most imaging applications, 

flat fielding would be optional. 

For full‐field imaging on a tight budget, the 

EdgeHD 925 is an excellent choice. It offers nearperfect 

on‐axis performance and outstanding 

images over a full 42 mm image circle. 



The 11‐inch EdgeHD is optimized to produce its 

sharpest images in green and red; at these wavelengths 

it is diffraction limited over roughly twothirds 

of the full 42 mm image circle. 

The relative illumination remains 100% across 

the central 16 mm, then falls slowly to 83% at the 

very edge of a 42 mm image circle. For pictorial 

images with an APS‐C digital SLR camera, flats are 

unnecessary. For monochrome imaging with an 

astronomical CCD camera, we always recommend 

making flat‐field images. 

On nights when the seeing achieves 1.5 arcseconds 

FWHM, star images shrink to 18 μm at the 

focal plane. On such nights, the EdgeHD 1100 

delivers fine images over a 30 mm image circle, 

and well‐defined stars over the full 42 mm field. 

The EdgeHD 1100 is a serious telescope. Its long 

focal length and large image scale give it the ability 

to capture stunning wide‐field images of deep‐sky 

objects with a large‐format CCD camera. 



In the matrix spot diagrams, note the tight cluster 

of rays in green light, and the well balanced 

spherochromatism in the blue and red. These spots 

are far better than would be spots from a fine apochromatic 

refractor of the same aperture! 

In green light, the EdgeHD 1400 is diffraction 

limited over a 28 mm image circle, although atmospheric 

seeing enables it to display its full resolution 

only on the finest nights. Relative illumination 

is 100% across the central 16 mm, and falls slowly 

to 83% in the extreme corners of a full‐frame 

35 mm image sensor. We have seen excellent 

results when the 14‐inch EdgeHD is used with a 

KAF‐16803 CCD camera over a 50 mm circle. 

The EdgeHD 1400 is a massive telescope, well 

suited to a backyard observatory or well‐planned 

away‐from‐home expeditions. Its long focal length 

and large image scale offer skilled imagers the 

opportunity to make images not possible with 

smaller, less capable telescopes. 


Appendix B: 

Technical Profile of the EdgeHD 

0.7× Focal Reducer Lens 

Perhaps the most useful accessory you can get for an 

EdgeHD telescope is a focal reducer. Although the long 

focal length is a great advantage in capturing detailed 

images of nebulae, galaxies, and especially of planetary 

nebulae, it also means the field of view is sometimes 

smaller than desirable, and the relatively slow focal 

ratio necessitate exposures that are rather long. We 

designed our 0.7× Focal Reducer to provide a field of 

view 1.4× larger angular diameter (giving you twice 

the sky area coverage) and halving the exposure time 

required to reach a given signal‐to‐noise ratio. If your 

passion is imaging large deep‐sky objects, imaging in 

Hα, SII, and OIII narrowband, or capturing the faint 

reflection nebulae often found around Barnard’s dark 

objects—or just cutting your exposure (and guiding) 

times down—the focal reducer is a “must‐have” item. 

Back in the days of film astrophotography, focal 

reducers came to be poorly regarded. Although they 

would shorten the focal length, they also produced 

fuzzy star images, had bad field curvature, and suffered 

from severe vignetting. But the days of film and 

ersatz focal reducers are gone. The modern EdgeHD 

focal reducer is the product of optical engineering and 

precision manufacturing on a par with the design and 

production of wide‐ and ultra‐wide field eyepieces. 

We designed two EdgeHD 0.7× Focal Reducers, one 

specifically tailored for the EdgeHD 1100 and the other 

for the EdgeHD 1400. With a clear aperture of 60 mm, 

each reducer contains five precision optical elements. 

To attain a level of performance worthy of the 

EdgeHD, the designs employ low‐dispersion lanthanum 

rare‐earth glass to control both chromatic and 

geometric aberrations. All ten optical surfaces are 

multi‐layer anti‐reflection coated to maximize light 

transmission, provide high‐contrast images, and to 

minimize image ghosting. 

The matrix spot diagram opposite shows that star 

The EdgeHD 0.7× Focal Reducer 

Figure B1. The EdgeHD 0.7× Focal Reducer is a fiveelement 

optical system that shortens the focal ratio 

of the EdgeHD 1100 to ƒ/7 and that of the EdgeHD 

1400 to ƒ/7.6 while maintaining sharp images across 

the full 42 mm field. This enables CCD imagers to 

reach the same signal-to-noise ratio on extended 

objects in half the exposure time, and brings even the 

faintest deep-sky objects within the range of your 

high-end digital SLR camera. 

images on‐axis are diffraction limited in green light, 

while rays at all wavelengths concentrated near the 

Airy disk. Even at the outer edge of the 42 mm image 

circle, green and blue rays are clustered tightly, while 

red shows only a weak flare. 

Both physically and mechanically, the 0.7× Reducer 

Lens is more than comparable to a top‐of‐the‐line 

wide‐field eyepiece. The CNC‐machined housing easily 

supports the full weight of your CCD camera or 

digital SLR camera without sag or movement. And for 

safe storage, each unit is provided with threaded metal 

covers for both the front and the back. 


Celestron EdgeHD 0.7× Reducer 


T-Ring Adapter 

Digital SLR 


1100 and 

1400 

5.75 inches 

146.05±0.5 mm 

0.7× Reducer 


T-system Spacer 

CCD Camera 

The matrix spot diagrams show that the 

bulk of rays cluster tightly in or near the Airy 

disk, with a diffuse scatter most strongly seen 

in the red. Plotted these at the same scale as 

those for the EdgeHDs, the spots demonstrate 

that the focal reducer’s star images are even 

smaller than those of the telescopes. 

For observers who wish to pursue faint nebulae 

in RGB or in narrowband, the 0.7× Focal 

Reducer is a valuable accessory that halves the 

necessary exposure time with no sacrifice in 

resolution or image quality. 


Imagine the thrill of seeing the first images from 

your Celestron EdgeHD! A quick glance at the 

whole image shows that you have captured your 

target’s faint outer extensions. Across the field, 

from one side to the other, star images are sharp, 

crisp, and round. As you process your image, fine 

details in the target object reveal themselves to 

you. Star clouds, delicate dust lanes, subtle HII 

regions...it’s all there, credit to your skill and the 

design of your EdgeHD telescope. 

The image shown here is a single monochrome 10- 

minute exposure taken with an Apogee U16 camera 

(KAF-16803 CCD chip) with a Celestron EdgeHD 

1400 telescope on a CGE Pro mounting. 

Image by André Paquette 

The Celestron EdgeHD technical white paper. Copyright © 2012 by Celestron. All rights reserved.

Celestron EdgeHD White Paper - First Light Optics

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